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Sept. 9, 2011 -- One or two doses of a vaccine that prevents cervical cancer may be just as effective as three doses, according to a new study from the National Cancer Institute. But investigators say more years of follow-up are needed to confirm the findings.

If confirmed, the research could have a major impact in poor resource regions like Africa and Central America where cervical cancer rates are highest. It may also be helpful in the U.S., where only about one in three eligible teens receive all three recommended doses of the human papilloma virus (HPV) vaccine.

The vaccine used in the study, the HPV 16/18 vaccine, prevents infection with the two strains of HPV that cause most cervical cancers.

Currently, recommendations call for three doses of the vaccine to be given over six months. Girls in the U.S. are typically vaccinated between the ages of 11 and 12. The vaccine can be given between ages 9 and 25.

HPV Vaccine: Less Is More?

The National Cancer Institute (NCI) study is the first to examine protection against HPV infection in women who received less than the recommended three doses of the vaccine, but that was not the intent of the research, NCI epidemiologist Aimee R. Kreimer, PhD, tells WebMD.

Women enrolled in the Costa Rica Vaccine Trial were supposed to get all three doses of the vaccine. But around 20% of the 7,466 enrollees were not able to have the full series due to pregnancy or for some other reason.

Four years after vaccination, Kreimer and colleagues examined HPV infection rates among women who received one, two, or three doses of the vaccine.

They found similar levels of protection against HPV 16 and HPV 18 in all three groups.

"The vaccine efficacy [effectiveness] with one or two doses looked to be quite the same as it was for all three doses," Kreimer says.

But she adds that at least a decade of follow-up will be needed to determine if the women who got less than the full series will remain as fully protected as those who got all three doses of the HPV 16/18 vaccine.

3 Shots Still the Gold Standard

Until then, it is not likely that the three-dose series recommendation will change in the U.S., she says.

"Three is the gold standard in the United States, and we require a very high level of evidence before we move away from the gold standard."

Ob-gyn Jennifer Wu, MD, of Lenox Hill Hospital in New York City, says she would need to see at least a decade of follow-up before she would be comfortable giving less than the full series of HPV shots to her patients.

The Costa Rican women in the NCI study were between the ages of 18 to 25 at enrollment. Wu says it is not clear if younger women, like those recommended for vaccination in the U.S., would respond as well to a less than full series of shots.

The study was published online today and will appear in the Oct. 5 issue of the Journal of the National Cancer Institute.

"These findings are certainly exciting, but the message is still that we need to give three shots for the best protection possible," she says.